1. Field of the Invention:
The invention relates in general to electrical distribution apparatus, and more specifically to electrical distribution apparatus having overvoltage or surge arresters.
2. Description of the Prior Art:
Electrical distribution apparatus, such as distribution transformers, must be protected from over-voltages or voltage surges, such as due to lightning. Pole mounted distribution transformers are inaccessible by the public, and thus surge arresters are quite naturally mounted on the outside of the transformer tank.
Pad-mounted distribution transformers used with underground distribution systems have a protective cover over the terminal or bushing compartment. Thus, with live-front designs, a surge arrester may be mounted external to the terminal wall, but within the protective cover. With dead-front designs, a surge arrester has been mounted in a plug-in elbow, with one arrester elbow for each primary bushing elbow. With loop feed, the high voltage connection is from a primary bushing well to an arrester bushing well, with the connection being made inside the tank. With radial feed, special dual type feed-through inserts for plug-in elbows are provided in which the high voltage connection from the bushing elbow to the associated arrester elbow is outside the tank.
Plug-in elbows and bushing wells are costly, so surge arresters have also been mounted inside the transformer tank, directly in the liquid dielectric, e.g., mineral oil. The oil-immersed type has the advantage of being protected from the environment external to the transformer tank, which environment has been a major cause of arrester failure over the years. The oil-immersed type, however, suffers many disadvantages. For example, the arrester may contaminate the liquid dielectric in the event the arrester is destroyed while accommodating a voltage surge. Also, the in-oil type is inaccessible, and thus not easily inspected or replaced. Some tanks are sealed, with no hand-hole cover, and even with a hand-hole cover, reaching blindly into the liquid dielectric to locate, remove and replace an arrester is time consuming and messy. Certain transformer surge and dielectric tests must be made without the surge arrester in the circuit. With the in-oil type of arrester, these tests are very costly since the arrester must be disconnected for the test and reconnected afterwards. Finally, a surge arrester is not required for every pad-mounted distribution transformer. For example, an arrester may be used at the initial line drop from the overhead to the underground, and at the end of the line. The end of the line, however, changes with system expansion. With the in-oil type it is a practical necessity to purchase all transformers with arresters.